If you have a bike reflector like the one in the images below, you can download and 3D print this holder for your AirTag. This seems to be a good hiding spot for the AirTag, less obvious than under the seat.
3D Printing & Assembly
To remove the mount from your reflector light, use a flat screwdriver to lift the end while sliding it out from the guide.
Once you print both STL files. The Top component will need to be glued in place. I used Gorilla Glue, its easy to work with and bonds PLA well. Make sure you clean your surfaces with some isopropyl alcohol and make suer both surfaces are pressed together until the glue sets. Used a flat plastic cap to press the glued components together.
Here are my print settings. As you know 3D printing is largely experimental so your result may vary from mine.
Of course if you do not want to go through the hassle of DIY there are similar products ready for purchase, here is an example from Amazon. Please note that by purchasing these linked products I may receive a commission that will go towards the support of this site and future 3D designs.